Interpretive Summary: Huanglongbing (HLB) is a highly destructive disease of citrus production worldwide. The disease is currently associated with the infection of a bacterium called ‘Candidatus Liberibacter asiaticus’. Since the HLB bacterium cannot be cultured in artificial media, a critical step in conventional bacteriology research, many biological and epidemiological unknowns remain. In this project, a region in the genome of the HLB bacterium was first found to have a high level of DNA sequence variation. Further investigation revealed that variation was associated with DNA mutation (insertion/deletion) at two locations. This variable feature was used to analyze a total of 262 ‘Ca. L. asiaticus’ strains from China and Florida. The results further confirmed the inter- and intra-continental variation among ‘Ca. L. asiaticus’ strains.

Technical Abstract:
Huanglongbing (HLB) is a highly destructive disease of citrus production worldwide. The disease is associated with ‘Candidatus Liberibacter asiaticus’, an alpha proteobacterium currently unculturable in vitro. Information about the biology and strain diversity of ‘Ca. L. asiaticus’ is limited, inhibiting the scope of HLB research and control of the disease. In this project, a region in the ‘Ca. L. asiaticus’ genome showing a high frequency of DNA sequence variation was investigated with a total of 262 ‘Ca. L. asiaticus’ strains (188 from China and 74 from Florida). Based on characteristic electrophoretic profiles of PCR amplicons generated by a specific primer set, eight electrophoretic types, defined as E-types, were found. Six E-types (A, B, C, D, E, and F) were found in China and four E-types (A, C, G, and H) were found in Florida. Overall, the China population consisted predominately of strains with E-type A (71.3%) and E-type B (19.7%). The bacterial population from Yunnan Province was the most diverse with five E-types. In contrast, only two E-types were detected in Guangdong where HLB was first described. The Florida population was predominated by strains with E-type G (82.4%). Sequence analysis indicated nucleotide insertions/deletions at two positions, one within an open reading frame and the other in the intergenic region downstream, accounted for the mosaic feature of the locus.